1. Field of the Invention
The present invention relates to a semiconductor wafer having a buried insulating film on which an active layer is formed and a semiconductor device formed from the wafer. In particular, the present invention relates to a SOI structure containing active layers and a bulk section, the active layers having different thickness depending on the purposes of semiconductor elements formed therein, the bulk section having a semiconductor element without a buried insulating film.
2. Description of the Related Art
Rapidly spreading portable information devices require high-speed, low-power-consumption semiconductor devices. Such semiconductor devices require an improved device structure as well as an improved system design and circuit design.
A semiconductor device formed on a bulk silicon substrate may have reduced power consumption if load capacity and supply voltage thereof are decreased. The bulk silicon substrate, however, deteriorates the operation speed of the device. High expectation is held regarding silicon on insulator (SOI) substrates, for forming semiconductor devices which simultaneously realize high-speed operation and low power consumption. A semiconductor device formed on an SOI substrate may simultaneously realize high-speed operation and low power consumption if parasitic capacitance between the device and the substrate or between wiring of the device and the substrate is reduced.
The SOI substrate is identified by an SOI structure encompassing an insulating layer and a monocrystalline silicon layer on the insulating layer. The SOI structure is capable of reducing junction capacitance and substrate bias effect without deteriorating operation speed. The SOI structure allows a supply voltage to be decreased to realize low power consumption. An SOI wafer is a wafer having the SOI structure. The SOI wafer is formed by a silicon wafer direct bonding (SDB) method or a separation by implanted oxygen (SIMOX) method. The SDB method bonds silicon wafers to each other with an oxide film interposed between them. The SIMOX method implants oxygen ions into a silicon wafer and carries out heat treatment to form a buried oxide film in the silicon wafer.
Semiconductor elements formed on the SOI wafer or SOI substrate are called SOI elements and are formed in the monocrystalline silicon active layer on the buried oxide film of the SOI substrate. The thickness of the active layer is dependent on the purpose of the SOI elements formed therein. If the SOI elements are CMOS elements operating at high speed and low power consumption, the thickness of the active layer is about 50 to 100 nm. If the SOI elements are high breakdown voltage elements, the thickness of the active layer is several micrometers.
A protective diode is a semiconductor element provided to a semiconductor device, to guide an over current from a connection pin to a substrate and to the outside of the semiconductor device, to thereby protecting an internal circuit of the semiconductor device. As such a protective diode, the buried oxide film in the SOI substrate is useless or obstructive and a bulk silicon substrate is more preferable than the SOI substrate.
In the SOI wafer or SOI substrate, the buried oxide film is uniformly formed to entirely cover the wafer. Namely, the buried oxide film has a uniform depth measured from the surface of the wafer, and the active layer has a uniform thickness over the wafer. It is difficult, therefore, to form SOI elements of different purposes on the same SOI substrate.
According to a first aspect of the present invention, a semiconductor wafer has a first element forming section, a second element forming section adjoining the first element forming section, and a third element forming section adjoining the second element forming section. The first element forming section has a first supporting substrate, a first buried insulating film formed on the first supporting substrate, and a first active layer formed on the first buried insulating film. The second element forming section has a second supporting substrate, a second buried insulating film formed on the second supporting substrate, and a second active layer formed on the second buried insulating film. The second active layer has a thickness being different from a thickness of the first active layer. The third element forming section has a third active layer.
According to a second aspect of the present invention, a semiconductor device has a first element forming section, a second element forming section adjoining the first element forming section, a third element forming section adjoining the second element forming section, and first, second and third semiconductor elements merged in the first, second and third element forming sections, respectively. The first element forming section has a first supporting substrate, a first buried insulating film formed on the first supporting substrate, and a first active layer formed on the first buried insulating film. The second element forming section has a second supporting substrate, a second buried insulating film formed on the second supporting substrate, and a second active layer formed on the second buried insulating film. The third element forming section has a third active layer. The second active layer has a thickness being different from a thickness of the first active layer.
According to a third aspect of the present invention, a method of manufacturing a semiconductor wafer includes preparing a monocrystalline silicon substrate, forming, on the substrate, a first ion-implantation protection film having an opening corresponding to a first element forming section of the substrate, selectively implanting oxygen ions through the opening into the first element forming section to a first depth, forming, on the substrate, a second ion-implantation protection film having an opening corresponding to a second element forming section of the substrate, selectively implanting oxygen ions through the opening into the second element forming section to a second depth being different from the first depth, heating the substrate to react the implanted oxygen atoms with silicon atoms.
According to a fourth aspect of the present invention, a method of manufacturing a semiconductor device includes preparing a semiconductor substrate, forming, on the substrate, a first ion-implantation mask having a first opening defining a first element forming section of the substrate, selectively implanting oxygen ions through the first opening into the first element forming section with a first projected range from a top surface of the substrate, forming, on the substrate, a second ion-implantation mask having a second opening defining a second element forming section of the substrate, selectively implanting oxygen ions through the second opening into the second element forming section with a second projected range from the top surface being different from the first projected range, heating the substrate to react the implanted oxygen atoms with silicon atoms so as to form a first buried insulating film under the first element forming section and a second buried insulating film under the second element forming section, and forming first, second and third semiconductor elements in the first and second element forming sections and a third element forming section, respectively. The third element forming section is defined as a part of the substrate neighboring to the second element forming section.